Very similar processes are utilized to manufacture a large variety of integrated circuit packages or chips. A starting substrate, e.g., a thin wafer of silicon or gallium arsenide, is masked, etched and doped during several process steps, with the type, number and order of the steps depending on the type of integrated circuit being manufactured, to form a number of dies or separate integrated circuits thereon. The dies are singulated or separated with a wafer saw and then packaged individually to form integrated circuit packages or chips.
Each die typically has a back that is devoid of circuitry and a front having integrated circuitry formed thereon. The integrated circuitry is accessible via die wire bonding pads that may be arranged in a variety of configurations on the face or edges of the die.
During the packaging process, each die is attached to a corresponding lead frame. The lead frames are processed in groups, typically in strips or in a continuous coil form. Each strip contains multiple, e.g., six, lead frames and is several, e.g., nine, inches long. A coil contains a larger number of lead frames because it is a much longer continuous strip of material. Each lead frame strip or coil typically is a metal frame designed to support several dies for packaging and 1o provide the leads for the final integrated circuit package.
A typical lead frame strip or coil is produced from metal sheet stock, e.g., a copper or nickel alloy, by subjecting the sheet stock to a stamping or photochemical etch process to form lead fingers, mounting paddles and side rails having pilot holes therein. The side rails and pilot holes therein facilitate transport and indexing of the lead frame strip or coil by automated packaging machinery. For example, a lead frame strip, individually, or a lead frame coil, as a whole, can be removably clipped to carrier belts that move the lead frame strip or coil through the various manufacturing stages.
Each lead frame has a plurality of lead fingers for connection to the die bonding pads. Each lead frame might further include a mounting paddle to which a corresponding die is attached. The paddle is usually about the same length and width as the die. Alternatively, each die can be attached to a lead frame with a tape comprising a nonconductive plastic or polymide carrier material having an adhesive on one or both sides.
If a mounting paddle is used, the paddle is typically downset or stamped to a plane below that of the rest of the lead frame prior to attaching the die so that the bottom of the attached die rests below the lead fingers on the lead frame. The downset paddle permits the attached die to be positioned so that the distance from the top of the die to the bottom of the paddle is roughly centered with respect to the plane of the lead frame. In this configuration, the amount of mold compound above the die roughly equals the amount below the die, which reduces the amount of stress on the bond wires from the flow of mold compound during the encapsulation process. Moreover, a paddle without a downset can cause the final chip to bow due to the shrinkage of the mold compound as it cures. The paddle downset also permits shorter bond wires, which reduce the inductance of the wires, reduce the probability of wire movement during encapsulation and reduce the amount of material, often gold, consumed for bond wires.
The lead fingers typically are spot plated with paladium, gold or silver along with the mounting paddle, in part because the paddle cannot be easily masked during the plating process. The conductive leads are plated to provide a metallic surface to which wires may be bonded, as a bond wire usually will not stick directly to lead frame material, such as copper or nickel or alloys thereof.
During a conventional packaging process, each die is attached to a mounting paddle with an adhesive layer, although thermoplastic, tape or other materials are also used. The adhesive layer typically is formed of an epoxy, acrylic, silicone or polymide material that is sandwiched between the back of the die and the mounting paddle.
Regardless of the method utilized to attach each die to its corresponding lead frame, the die bonding pads are electrically connected to the lead fingers of the lead frame with fine bond wires that can be formed using an ultrasonic bonding method. Following the application of a polymide protective layer to the face of each die, each die, its corresponding bond wires and a portion of its corresponding lead fingers are encapsulated in a protective plastic casing via a pressurized, resin-injection mold process, resulting in an integrated circuit package. The casing protects the die from environmental damage, e.g., breakage, physical abuse and contamination by moisture and chemicals. A portion of the lead fingers extend beyond the casing to permit electrical connections between the encapsulated die and a printed circuit board or other external circuitry for eventual use in an application.
After encapsulation, the packages require a deflashing process for removing excess molding material from the casing exterior. A trim and form operation then singulates the resultant interconnected packages and bends the protruding lead fingers of each package into an appropriate shape.
As the demand for thinner packages increases, it becomes desirable to make the die-lead frame assembly thinner, which can be accomplished by reducing the thickness of the lead frame. However, thinner lead frames exacerbate the problems of lead movement during processing. Thinner lead frames also can result in package leads that are easily damaged or misaligned. Lead movement is especially problematic after wire bonding. The lead fingers are relatively long for their thickness and, therefore, can bend and move around quite easily. As the assembly is transported to the location of the encapsulation step, the wire connections can be broken and shorts can occur. Lead movement increases as the thickness of the lead frame decreases.
To alleviate this problem, manufacturers often apply tape across the lead fingers prior to die attachment to immobilize the lead fingers so that lead movement is greatly reduced or eliminated. The tape comprises a nonconductive plastic or polymide carrier material having an adhesive on one side. Usually, multiple pieces of tape are applied to each lead frame, with the number of pieces and location and shape of each piece depending on the configuration of lead fingers to be taped. The tape should be applied at locations where the need for immobilization is greatest, which will vary greatly depending on the particular lead frame design being used.
The tape can be applied to the lead fingers by an automated tape applicator that utilizes a series of rollers to remove the backing from a roll of tape, carry the tape strip over a lead frame strip or coil, stamp an appropriately-shaped piece of tape out of the tape strip, press the adhesive side of the tape piece against the lead fingers of a lead frame and activate the adhesive on the tape piece, e.g, by heating the tape piece and underlying lead frame strip or coil. Multiple passes through the tape applicator may be required if multiple pieces of tape are to be applied to each lead frame.
During encapsulation, however, the solid lead frame tape restricts the flow of mold compound between the lead fingers and other features of the lead frame where the tape is present. As a result, the protective casing is more prone to delamination, i.e., separation of the mold compound from either side of the lead frame. Delamination is most likely to occur where the solid lead frame tape forms a continuous, i.e., uninterrupted, path around the center of the integrated circuit package.